In production of steel, a sliding nozzle device is employed to control a flow rate of molten steel to be discharged from a molten metal vessel, such as a ladle or a tundish. In the sliding nozzle device, two or three refractory plate bricks having a nozzle hole are used. The plate bricks are disposed in superimposed relation and adapted to be slidably moved with respect to each other while applying a surface pressure therebetween, to adjust an opening degree of the nozzle hole so as to adjust the flow rate of molten steel.
Typically, the plate brick is made of an alumina-carbon material, and classified into a burned type and an unburned type according to a production method thereof. The burned type is used as a relatively large-size plate brick, because it is excellent in strength although a cost thereof becomes higher than that of the unburned type.
In many cases, a usable life of the plate brick is determined by damage imposed on a sliding surface serving as an operating surface. It has been considered that a causal factor of the damage of the sliding surface of the plate brick (hereinafter “surface roughing”) includes a loosening of the microstructure due to thermal shock, a chemical reaction with molten steel, abrasion due to sliding, and oxidation of carbon bonds.
As a technique for improving surface-roughening resistance, for example, with a view to improving thermal shock resistance, a refractory raw material having a low thermal expansion coefficient, such as zirconia-mullite or alumina-zirconia, is used. Further, with a view to improving corrosion resistance, it is studied to use a high-purity and dense raw material less likely to react with slag, such as fused alumina, and adequately select/set a type and an amount of carbon. With a view to improving sliding abrasion resistance, for example, a technique of adequately setting a particle size distribution of a raw material to obtain a dense microstructure and therefore achieve higher strength, and a technique of improving polishing accuracy of the sliding surface, are implemented. With a view to preventing oxidation, addition of an antioxidant, such as silicon carbide, boron carbide or aluminum nitride, is performed.
As above, with a view to improving the surface-roughening resistance of the plate brick, a combination of various refractory raw materials is studied. In parallel therewith, improvement in a production method, such as a burning condition, is also studied.
For example, as a typical production method for a burned-type plate brick, there has been known a method which comprises: adding an organic binder to a refractory raw material mixture containing metal such as silicon; kneading them, forming the kneaded mixture into a shaped body; and burning the shaped body at 1000° C. or more under a condition that it is placed in a container filled with coke particles. A bonding microstructure of this burned-type plate brick is comprised mainly of a carbon bond and a silicon carbide bond, wherein an oxide-based sintered bond and others are entangled therewith. The carbon bond has a low elastic modulus and excellent thermal shock resistance, as compared with a ceramic bond, and the silicon carbide bond has an effect of imparting strength and oxidation/abrasion resistances, so that the bonding microstructure exhibits excellent properties capable of suppressing damage during use. However, if silicon carbide is oxidized, it is transformed into SiO2, which will react with a foreign component, such as FeO or CaO, to form a low melting-point material, causing deterioration in corrosion resistance. Moreover, there is a problem that SiO2 is likely to vanish as SiO gas under high temperatures, and thereby it is difficult to sustain the above effect over long hours of use.
In the burned type, it is also known to use aluminum instead of silicon. For example, Patent Document 1 (JP 57-27971 A) discloses a technique of forming a shaped body using a refractory raw material mixture containing aluminum, and burning the shaped body within carbon in a reducing atmosphere to obtain an aluminum carbide or aluminum nitride-based reaction-sintered bond. It is described that an obtained plate brick is excellent in mechanical strength and corrosion resistance as compared with the conventional carbon bond-type.
However, when a plate brick of a type which includes an aluminum nitride bond based on addition of aluminum, as disclosed in the Patent Document 1 (JP 57-27971 A), is burned within carbon in a reducing atmosphere, a large amount of aluminum carbide is formed, which causes a problem of deterioration in hydration resistance. Specifically, in cases where a plate brick is detached for reason of operational schedules or the like and then reused, it will be left in a storage area for a certain period of time until subsequent use. Thus, the aluminum carbide is hydrated by water vapor in ambient air, to cause a problem of deterioration in strength and durability of the plate brick. Further, in cases where a plate brick is collected after use and then recycled, the collected plate brick is re-subjected to grinding or machining. Thus, the aluminum carbide is hydrated by water during the machining, to cause deterioration in strength and durability of the plate brick. The Patent Document 1 (JP 57-27971 A) also discloses a technique of burning the shaped body in a nitrogen gas atmosphere. However, if the burning is performed simply in the nitrogen gas atmosphere, the formation of aluminum nitride will be hindered by gases, such as CO and CO2, in volatile matter generated by decomposition of a binder, such as a phenolic resin, contained in the shaped body as an object to be burned. Thus, the effect of improving the surface-roughening resistance of a plate brick becomes insufficient, so that it cannot expect to obtain the effect at a desired level.
As for the burning condition, Patent Document 2 (JP 2003-171187 A) discloses a technique of burning and weakly oxidizing a shaped body in an atmosphere having an oxygen gas partial pressure of 10 to 10000 ppm.
However, even if the plate brick including an aluminum nitride bond based on addition of aluminum is burned under the burning condition disclosed in Patent Document 2 (JP 2003-171187 A), the formation of aluminum nitride will be hindered by gases, such as CO and CO2, in volatile matter generated by decomposition of a binder, such as a phenolic resin, contained in the shaped body as an object to be burned, as mentioned above. Thus, the yield of aluminum nitride is reduced, and thereby the oxidation resistance (surface-roughening resistance)-improving effect cannot be obtained at a satisfactory level.
Moreover, in the burning condition disclosed in the Patent Document 2 (JP 2003-171187 A), the presence of CO and CO2 is permitted (see the paragraph [0022] in Patent Document 2 (JP 2003-171187 A)). However, if CO and CO2 exist in the burning atmosphere in a large amount, there is a problem that the formation of aluminum nitride based on a reaction between nitrogen gas and aluminum is hindered.
As above, although the surface-roughening resistance of a plate brick is being improved by a combination of raw materials and a burning condition, the conventional techniques have the above problems. Thus, even now, a key factor of the usable life of the plate brick is still surface roughening.